Tenured faculty need your attention too-perhaps not in as concentrated a way as untenured faculty, but they will also benefit from encouragement, support, and attentive listening. There is still the review for promotion to tenure, or the post-promotion review, but even with these, there is a large shift in one's relationship to a faculty person after tenure because the role of evaluation is so diminished, given the job security of tenure.
The vitality of a department depends on the continued energy, productivity, and intellectual edge of all faculty. Make an effort to stay interested in the teaching and research/creative work of tenured faculty. Offer to look at work in progress. If the department has a colloquium, encourage them to participate. Encourage them to apply for grants and to continue to attend conferences. If you see that there may be an issue with their keeping up in the field and/or with technological developments related to the field or to teaching, give encouragement; you might even offer to engage together in some aspect of new developments. Talk with them about their agenda for the future, and do all that you can to facilitate it. Be aware that the life-course continues to have its ups and downs, even after tenure. Midlife crises or significant transitions are normal. Just when one thinks everything is settled, the ground can shift, whether in one's personal life or in professional interests. Be as supportive as you can during these changes. You can also consult with the Dean about any concerns you have; he or she can play an important supporting role-to you, to the faculty member, or to both.
In whatever stage they are, tenured colleagues are a great resource. They might have been chair before you-ask for their counsel and help. They may be the chair after you-involve them in decisions and tasks so that they not only are helping you but are learning valuable information and skills for the future.